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SWAT officers on the streets of Boston after the explosions which the White House said would be handled as an ‘act of terror’. (Picture: Reuters /Dominick Reuter)

The FBI is hunting the bomber or bombers behind the deadly blasts in Boston amid conflicting theories that Islamic extremists or domestic terrorists were responsible.

Bill Bratton, a former Boston police chief now based in London, said there are ‘no shortage of potential suspects’ behind the explosions.

Federal investigators said no one had claimed responsibility for the bombings on one of the city’s most famous civic holidays, Patriots Day, which commemorates the anniversary of the first battles in the US war of independence and may have inspired homegrown extremists.

Monday was also the day to file tax returns which could motivate anti-government activists.

A European security official said initial evidence indicates that the attacks were not the work of suicide bombers.

‘So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely,’ said the official.

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The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the marathon bombings.

A military vehicle patrols a few blocks from the Boston marathon finish line following the bombings (Picture: EPA/Matt Campbell)

The bombs reportedly contained ball bearings which mutilated many runners and spectators.

He added: ‘A bomb with ball bearing seems home made, the type of thing you would build if you don’t really know how to build a bomb, perhaps with some instructions or guide on the internet.’

Explosives with ball bearings have previously been used by insurgents in Iraq and Afghanistan.

At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: ‘This is something I’ve never seen in my 25 years here … this amount of carnage in the civilian population. This is what we expect from war.’

The FBI’s investigation is in full swing with authorities serving a warrant on a suburban Boston home and appealing for any private video, audio and still images of the blasts that killed three and wounded 140.

President Barack Obama is due to be briefed on the explosions on Tuesday by FBI director Robert Mueller. White House officials and investigators said it was too early to say whether the Boston attacks were carried out by a foreign or homegrown group or to identify a motive.

A flag flies over the finish line as medical workers aid injured people following an explosion at the finish line of the 2013 Boston Marathon (Picture: AP)

A Boston police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon (Picture: AP)

The blasts occurred close to the Boston marathon’s finish line (Picture: AP)

A runner is comforted in the wake of the Boston marathon explosions (Picture: Reuters)

Medical workers aid injured people at the finish line (Picture: AP)

Police clear the area at the finish line of the 2013 Boston Marathon (Picture: AP)

Medical workers aid an injured man (Picture: AP)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon (Picture: AP)

An unidentified Boston Marathon runner is comforted as she cries in the aftermath of two blasts (Picture: AP)

A runner embraces another woman near Kenmore Square after two bombs exploded (Picture: Getty)